The disclosure relates to screw compressors. More particularly, the disclosure relates to lubrication of screw compressors.
Screw-type compressors are commonly used in air conditioning and refrigeration applications. In such a compressor, intermeshed male and female lobed rotors or screws are rotated about their axes to pump the working fluid (refrigerant) from a low pressure inlet end to a high pressure outlet end. During rotation, sequential lobes of the male rotor serve as pistons driving refrigerant downstream and compressing it within the space between an adjacent pair of female rotor lobes and the housing. Likewise sequential lobes of the female rotor produce compression of refrigerant within a space between an adjacent pair of male rotor lobes and the housing. The interlobe spaces of the male and female rotors in which compression occurs form compression pockets (alternatively described as male and female portions of a common compression pocket joined at a mesh zone). In one implementation, the male rotor is coaxial with an electric driving motor and is supported by bearings on inlet and outlet sides (ends) of its lobed working portion. Similarly, the female rotor may be supported by bearings on inlet and outlet sides of its lobed working portion. There may be multiple female rotors engaged to a given male rotor or vice versa.
When one of the interlobe spaces is exposed to an inlet port, the refrigerant enters the space essentially at suction pressure. As the rotors continue to rotate, at some point during the rotation the space is no longer in communication with the inlet port and the flow of refrigerant to the space is cut off. After the inlet port is closed, the refrigerant is compressed as the rotors continue to rotate. At some point during the rotation, each space intersects the associated outlet port and the closed compression process terminates. The inlet port and the outlet port may each be radial, axial, or a hybrid combination of an axial port and a radial port.
In operation, the pressure difference across the compressor produces a thrust load on the rotors. The pressure at the discharge end of the rotors will be higher than that at the suction end producing a net thrust force from the discharge end toward the suction end. To address such forces, the rotors may typically have a thrust bearing at one end. In a number of compressors, exemplary thrust bearings are unidirectional in that they absorb or react thrust loads in only one direction. This direction is selected to absorb the operational thrust load from the discharge end toward the suction end (hereinafter referred to as upstream thrust for ease of reference).
In particular situations such as unintended loss of power, the upstream thrust force is lost. The rotors may still have rotational inertia. The loss of the thrust force may, however, allow one or both rotors to shift downstream bringing the discharge end face of the lobed portion of such rotor into contact with an adjacent face of the outlet case (e.g., an upstream face of a discharge bearing case along a discharge end plane). This contact may be damaging.
One solution to such problems is to add an additional thrust bearing positioned to take up downstream thrust loads before the rotor end contacts the case. For example, this may involve mounting to one or both rotors an additional unidirectional thrust bearing generally similar to but oppositely oriented relative to the thrust bearing that takes up the upstream thrust loads. However, this adds cost and potentially compromises efficiency.